Corsair like many of the best are very conservative w/ their power numbers and can output at the max all day long. Some say a 750 corsair = 1k mid grade unit = 1.2 cheap unit....but that might be overstating things a smidgen.

With that being said if YOU are not comfortable with "only" the 750....get the 850. IMHO peace of mind is priceless. ;)

IIRC Hx750/850 are CWT's and thus GREAT kit. Good choice.
I've used both in many builds. :)

I would say 750w=900w=1.1KW. If you look on the label, Corsair PSU's usually put out a combined wattage (all rail wattages combined) of at least 50-150w over the rated wattage. Add on peak load to that and you would be able to game (not fold, as it doesn't spike power consumption much) while drawing around a 850-900W peak load on a 750W PSU.